Process for the solid state polycondensation of linear polyesters

ABSTRACT

A separating or anticaking agent consisting of spherical glass particles having a diameter of 3 to 30μ is added to polyester resin granulates to prevent agglomeration during solid state polycondensation in a plug-flow reactor. Thin-walled articles formed from the end product are characterized by high strength, high transparency, crystal clear color, good gas impermeability and a smooth surface.

BACKGROUND OF THE INVENTION

The invention relates to a process for the solid-phase polycondensationof linear polyesters, especially of polyethylene terephthalate, whichare continuously moved through a reaction vessel in granulated form inan inert gas stream at temperatures of 30° to 5° C below the meltingpoint, sticking of the granulates being prevented by addition offine-particle glass as an anticaking agent.

Polyethylene terephthalate can be condensed in the melt in autoclaves upto an intrinsic viscosity of maximally 0.7 and in reactors of specialconstruction up to an intrinsic viscosity of 0.9, maximally 1.0, withoutsuffering any substantial thermal damage. For certain purposes of use,however, a higher molecular weight is required, i.e., a higher intrinsicviscosity than is possible by condensation in the melt. In varioustechnical areas there is an increasing need to replace inorganicmaterials with synthetic substances of especially high strength. Byreason of a large number of favorable properties, including, forexample, the lower density, there are many uses for high-molecularsubstances such as polyethylene terephthalate, polybutyleneterephthalate, polyamides, etc. The strength of these resins depends onthe molecular weight. An increase in the molecular weight over thelimits set by the dynamic viscosity in the condensation in the melt canbe achieved essentially only by a solid-state polymerization orpolycondensation. For a number of uses polyethylene terephthalate havingspecial properties is required, as for example, in the case ofthin-walled packaging for foods. The requirements for this product willbe discussed in detail below.

A number of continuous processes for solid-state polycondensation ofpolyethylene terephthalate are taught in the prior art.

German laid-open application OS No. 1,770,410 describes such a process,in which the granulate is moved by gravity in a plug flow. To be sure,as little as possible agglomeration is supposed to occur there, but itis proposed, through the admission of cold gas at the outlet of thereactor, to bring about a bursting apart of the particles, whichnevertheless may be stuck together. The sticking obviously cannot beavoided completely. The process has the substantial disadvantage thatthe emergence of vapors such as the diol formed in the condensation isprevented at the agglutinated places because of the increase in thediffusion path length. As a consequence differing degrees ofcondensation occur in the various parts of the individual granulateparticles, so that the end product has a very broad spectrum ofviscosity or chain length.

During polycondensation an increase in density and a shrinkage in volumeoccurs and glacierlike cracks develop in the mass into which thegranulate plunges from above. This results in the disadvantage that theotherwise plug-flow profile in the reactor space is disturbed, so thatthere is formed a polycondensation product of widely varied solutionviscosity.

Discontinuous processes in which the granulate to be treated isprevented from agglutinating within a closed vessel by mechanicalturning, for example, by agitating mechanisms, are not considered here,since other significant disadvantages arise, such as high powerconsumption and labor requirements, as well as a long reaction time. Thetrend, therefore, is toward carrying out the solid-statepolycondensation continuously. Solid-state polycondensation processeswhich operate under high vacuum are excluded because of the problems ofsealing the reaction space for continuous operation. Further, whereleakage occurs there is a danger of the penetration of oxygen into thereaction chamber, whereby the polymer suffers damage by oxidation.

PRIOR ART

To avoid the above-described agglutination it has been proposed to carryout solid-state polycondensation in a fluidized layer under action withhot inert gases, for example, nitrogen. The required high residencetimes which, depending on the polycondensability and molecular weight ofthe starting material, desired molecular weight increase, particle size,etc., can amount to up to about 20 hours, results in an extremely hightechnical expenditure both with respect to the investment and energyrequirements. Moreover fluidized layer processes lead to unfavorableresidence-time spectra, and, therefore, can be operated onlydiscontinuously.

In other known processes rotary pipes are used which are heated directlywith hot inert gases. They, too, because of the long residence times,require a great expenditure in apparatus, in which connection thereexists the additional problem of sealing the rotary pipe with respect tothe stationary connections which is hardly possible to do reliably atthe high operating temperatures. The residence time behavior does notcorrespond to the plug flow.

From the standpoints of the space-time yield, the investment costs andthe energy requirements, a pipe operating as a "moving bed reactor",preferably with round cross section, is especially favorable. Inoperation, however, the agglutination tendency of the granulatedescribed above creates great difficulties, since the polyestergranulates, regardless of the starting viscosity and the degree ofcrystallization, have a marked tendency to agglutinate. It wasascertained that in the temperature range suitable for solid-statepolycondensation, between 30° and 5° C below the melting point, thereare virtually no conditions under which the polyester granulates can bemoved without agglutination.

To avoid agglutination a known practice is to add so-called separatingor anticaking agents to the granulate to be condensed. Known separatingagents may range from liquids to solids, including solutions,dispersions, emulsions, etc. In German laid-open application OS No.2,117,748 there are mentioned as solid or powder-form separating agentssilicon dioxide and silicates. That specification makes no disclosure ofgeometry of the individual powder particles so that it is presumed thatany form of the powder particles, including the irregular forms thatarise in grinding processes, are deemed suitable. It has been found,however, that an arbitrary form of the powder does not by any meanssuffice for all uses of the end product because the separating agent asa rule remains in the end product.

Glass powders have also been disclosed as separating agents. However,these are subject to the same considerations as the powdered separatingagents mentioned in German laid-open application No. 2,117,748.

To illustrate the inadequacy of known separating agents, consider thefollowing series of criteria prescribed for thin-walled packages ofpolyethylene terephthalate. First of all, the tensile strength must beas high as possible, the degree of polycondensation should be as high aspossible, so that the wall thickness of the package can be kept as thinas possible for the purpose of a low consumption of material. Thispresumes a high intrinsic viscosity above 1.0. Further, the polyethyleneterephthalate should be colorless, i.e., there must not be admixed anycoloring substance or substances which through thermal decompositionlead to a yellowing or favor the thermal decomposition of the polymer.Futhermore, the polyethylene terephthalate should be crystal clear,which precludes the addition of any clouding substances. In addition,crystallization in the final shaping must be suppressed, which meansthat crystallization-accelerating additives are not usable as separatingagents. Also, it is frequently required that the package of polyethyleneterephthalate be gas tight. There are cases conceivable in which athin-walled shaped body has to withstand an internal pressure of 9atmospheres of carbon dioxide without pressure loss over several weeks.For such purposes additives which promote the porosity of the polymersmust be excluded, such as, for example, coarse-grained separatingagents, or those that react chemically with the polyester and therebytend to form agglomerates. Agglomerate-forming substances trap gases andare just as suitable as substances that give off gases on heating.Further, the polyethylene terephthalate end product should have a smoothsurface, for optical reasons and for reasons of cleanliness orsterility. This requirement excludes separating agents by which thesurface of the unshaped polymer is affected in the sense of roughening.Unusable, for example, are aerosil, gypsum, asbestos, silicates, etc.For the packing of foods, the polyethylene terephthalate must also bephysiologically unobjectionable, i.e., the separating agent must neitherbe poisonous nor influence the taste of the package contents, forexample, by an odor of its own. Finally, the packaging material must beboth thermally and hydrolytically stable, so that the high degree ofpolycondensation remains preserved during the final shaping. Thisexcludes separating agents which lower the stability or increase the-COOH concentration.

Underlying the invention, therefore, is the problem of providing aprocess for solid-state polycondensation of linear polyesters using aseparating agent by which the above-mentioned criteria are met.

THE INVENTION

The solution to the problem posed is accomplished by admixing with thegranulate before the granulate enters the reactor, 0.2 to 5% by weightof a glass powder of substantially spherical form with a diameterbetween 3 and 30μ.

It was found, surprisingly, that glass powder in the form of spheres inthe indicated range of diameters satisfies all the requirements set. Theindicated diameter range is of importance to achieve an ideal separatingeffect. Spheres with appreciably larger diameter were easily displacedfrom the contact surfaces between the granulate particles. Spheres withappreciably smaller diameter than 3μ are easily completely impressedinto the polyester granulate when the surface of the granulate reachesthe softening point. Impression of the spheres is due to the staticpressure of the column of granules and in consequence the efficiency ofthe spheres as anticaking agent is significantly reduced or completelyeliminated. The expression "spherical" includes also a particle formthat is very nearly spherical so that the same effect occurs. While thespherical form is achieved by way of the softening state of the glass,an approximately spherical form can be generated, for example, bymechanical reworking in a ball mill of irregular-shaped particlesproduced by a grinding process. Glass powders with only approximatelyspherical form sell for a lower price than more perfect spheres. In anycase the glass powder useful in the invention is different from glasspowder produced by a pure grinding process and which has an irregularsplinter-shaped form.

Glass balls, in contrast to prior-known separating agents, behaveadvantageously as follows: spheres, in the melting up of the polyesterproduced by solid-state polycondensation, are enveloped by the meltwhich wets the particles on all sides. There is no loss of strength andno increase in the gas permeability of the packages, as would be thecase with sharp-edged glass splinters. Irregularly-shaped glass powder(grinding product) holds air bubbles on the surface and, therefore, doesnot bond faultlessly with the polyester melt.

While irregular-shaped powder tends to agglomerate with inclusion of airand in this manner can reduce the wall thickness of the end product tothe point where it becomes gas-permeable and loses appreciable strength,spherical glass powder has no tendency at all to form agglomerates and,therefore, is free of the disadvantages thereof.

Spheres form on the surface of the polymer granulate in exactlymonoparticular layers, so that a uniform coating of the granulate ispossible with a minimal quantity of separating agent. A smaller additionof separating agents improves the transparency of the end product, whichmatter will be gone into in more detail below.

Spherical glass powder, in contract to irregular glass powder, leads toa smooth surface in the end product even in areas not lying on a shapingwall. Hence a better appearance of the end products is achieved.Moreover, cleaning and sterilization of the surface is facilitatedconsiderably.

Spheres have the smallest possible surface for a given volume. For thisreason the phase boundary surface between the matrix of polyester andthe separating agent is minimal. Aside from the extraordinarily goodbonding between the matrix and separating agent, even with a severlydeviating index of refraction, clouding occurs only to a very slightextent. The clouding is a function of the size of the phase boundarysurface between the matrix and the incorporated separating agent, orother fillers.

Spheres, unlike other, irregularly shaped additives, do not acceleratethe crystallization of the polyethylene terephthalate or at least notmeasurably. This property of the spherical separating agent is extremelyimportant, because only amorphous polyethylene terephthalate is crystalclear. It is not possible to produce shaped bodies of high transparencyif crystallization-accelerating substances are present in the polymer.

An advantage of glass not to be underestimated is that, in contrast toother separating agents mentioned, it can be produced with differingindices of refraction. It is possible in this manner to approximate therelative index of refraction n_(rel) to the value 1, so that theclouding effect can be markedly reduced or eliminated entirely. Notationn_(rel) is the quotient of n_(a) and n_(p), n_(a) being the index ofrefraction of the separating agent and n_(p) the index of refraction ofthe polyethylene terephthalate.

The size of the polyester granulate is not critical but is selectedexpediently between 1 and 5 mm, preferably between 2 and 4 mm, maximumdimension (length). With a smaller granulate the polycondensation timeis shorter because of the shorter diffusion path within particles. Onthe other hand, the effectiveness of the separating agent must begreater and vice versa. The invention is explained in more detail in thefollowing examples and comparative examples. Examples 1-5 are not inaccordance with the invention.

EXAMPLE 1

Polyethylene terephthalate granulates of cylindrical form, having alength of 2.5 mm, a diameter of 2 mm, an intrinsic viscosity of 0.55, anindex of refraction of N = 1.64 and a density of 1.40 (corresponding toa degree of crystallization of 53 to 55%) was added continuously to asolid-state poly-condensation reactor. The polycondensation reactor wasa vertically-standing, tempered cylinder, having a height eight timesits diameter.

Nitrogen at 240° C was blown into the reactor countercurrent to thepolyethylene terephthalate flowing continuously from above. Thethroughput of the polyethylene terephthalate was adjusted to thedimensions of the reactor so that the residence time in the reactionspace amounted to 8 hours. Initially the granulate was discharged fromthe reactor with an intrinsic viscosity of 1.05. Gradually, however, thegranulate agglutinated in the reactor space to such a degree that acontinuous discharge was no longer possible.

EXAMPLE 2

The experiment of Example 1 was repeated using nitrogen at a temperatureof 230° C. After about 4 hours of operation the polyethyleneterephthalate granulate was agglutinated to such a degree that thedischarge took place irregularly and later stopped entirely.

EXAMPLE 3

The experiment according to Example 2 was repeated under the sameprocess parameters, except the polyethylene terephthalate granulate hada density of 1.43 (corresponding to a crystallization degree of ca. 70%)and an intrinsic viscosity of 0.85. After 10 hours of operation thecontents of the reactor were agglutinated to such a degree that thedischarge of the reaction product came to a standstill.

From the above tests it was concluded that solid-state polycondensationin a "moving bed reactor" is not feasible without addition of separatingagents.

EXAMPLE 4

Polybutylene terephthalate granulate of cylindrical form having a lengthof 3 mm and a diameter of 2. to 2.5 mm, and an intrinsic viscosity of0.72 was supplied continuously at a temperature of 150° in a dry state(water content less than 0.01% by weight) to a solid-statepolycondensation reactor. The reactor was a vertically-standing temperedcylinder, the height of which was eight times the diameter. Polybutyleneterephthalate was fed into the reactor continuously from the topdownward countercurrent to the flow of nitrogen at a temperature of 215°C. The throughput of the polybutylene terephthalate was matched to thedimensions of the reactor so that the residence time in the reactionspace amounted to 6 hours. Initially granulate was discharged from thereactor with an intrinsic viscosity of 1.2. After about 30 hours ofoperation agglutinated particles appeared in the discharged polyester,and the viscosity of the discharged product spread over a range of 0.15units, indicating a differing residence time in the reaction spacebecause of agglutination.

EXAMPLE 5

Polyethylene terephthalate granulate of the nature mentioned in Example1 was powdered in a drum mixer with 0.8% by weight of aerosil (finelygranular silicic acid) and then, analogously to Example 1, subjected toa solid-state polycondensation. The product discharged from the reactorhad an intrinsic viscosity of 1.04. Since no agglutination of thereactor content occurred, the process could be operated continuously.

The polyethylene terephthalate granulate produced in this manner wasworked in a conventional manner and by means of conventional equipmentinto films. By reason of the at least partially-agglomerated aerosil andthe unavoidable crystallization of the polyethylene terephthalate, thefilm was cloudy. The surface of the film was rough, and the gastightness was only 90% of that of a comparable film without aerosil asseparating agent. Because of the tendency to form agglomerates aerosilwas deemed to be an unsatisfactory separating agent.

EXAMPLE 6

Polyethylene terephthalate granulate of the type and quality mentionedin Example 1 was powdered in a drum mixer at 130° C with 1.0% by weightof a spherical glass powder with a diameter of 5 to 25μ and an index ofrefraction of n = 1.51 and, analogously to Example 1, subjected tosolid-state polycondensation during an 8 hour residence time.

The end product discharged from the reactor had an intrinsic viscosityof 1.05 and an index of refraction of n = 1.72. Agglutinations in thereactor were not detected, and the operation was continuous.

The polyethylene terephthalate produced in this manner was formed inconventional manner into thin-walled shaped parts, which showed only aslight clouding with fully smooth surface. The gas-tightness wasequivalent to comparable shaped parts produced without separatingagents.

It was determined that the use of glass powder in spherical formaccording to the invention produced good end products meeting allrequirements.

EXAMPLE 7

Polyethylene terephthalate granulate of the same quality as in Example 1was powdered, analogously to Example 6, with a spherical glass powdermade from light flint glass with an index of refraction of n = 1.72. Thesphere diameter ranged between 5 and 20μ. The solid-statepolycondensation was carried out analogously to Example 1 during an 8hour residence time. The granulate discharged from the reactor has anintrinsic viscosity of 1.05. No agglutinations were found in thereactor.

From this product films were produced which were crystal clear and had asmooth, shiny surface after stretching. These films had the samestrength and gas tightness as films of the same thickness withoutseparating agents. By making the index of refraction of polymer equal tothat of the separating agent a further advantage is achieved, namely,that of crystal clear transparency.

EXAMPLE 8

Polybutylene terephthalate granulate of the same quality as in Example 4was powdered in a drum mixer at 130° C with 0.8% by weight of aspherical glass powder with a diameter between 5 and 25μ, and,analogously to Example 4, subjected to solid-state polycondensation. Theend product discharged from the reactor after 6 hours residence had anintrinsic viscosity of 1.20. Agglutinations in the reactor were notdetected. The operation was continuous. The polybutylene terephthalateproduced in this manner was formed in conventional manner intothin-walled shaped parts which were only slightly cloudy with acompletely smooth surface. The gas tightness was equivalent to that ofcomparable shaped parts produced from polymer without separating agents.

It was determined that the use of glass powder in spherical formaccording to the invention led to surprisingly good end products.

It is to be understood that the embodiment of the invention which hasbeen described is merely illustrative of one application of theprinciples of the invention. Numerous modifications may be made to thedisclosed embodiment without departing from the true spirit and scope ofthe invention.

We claim:
 1. Process for solid-state polycondensation of linearpolyesters in granulated form comprising moving the granulate in aninert gas stream at temperatures of 30° to 5° C below the melting pointof the polymer continuously through a reaction vessel, and preventingagglutination of the granulate by an addition of fine-particle glass asa separating agent, characterized in the prior to entering the reactorthere is admixed with the granulate 0.2 to 5% by weight of a glasspowder having particles of substantially spherical form with a diameterbetween about 3 and 30μ.
 2. Process according to claim 1, in which thepolyester granulates range in size between about 1 and 5 mm maximumlength.
 3. Process according to claim 1 in which the spherical glassparticles have a diameter between about 5 and 25μ, and the polyestergranulates are between about 2 and 4 mm maximum length.
 4. Processaccording to claim 1 wherein the indices of refraction of said sphericalparticles and said condensed polyester are substantially the same.